Somalia's al-Qa'ida-linked rebels have said that they will kill the French intelligence agent they hold hostage.

The Islamist extremists said France signed the death warrant of a French intelligence agent by launching a rescue operation last weekend that failed to free him.

The militant group al-Shabab has held the French agent, Denis Allex, (pictured) since July 2009. Al-Shabab said that it decided to kill Mr Allex in retaliation for the operation. Two French soldiers and 17 Somalis were killed during the rescue attempt, French officials say.

French officials believe Mr Allex is already dead. Al-Shabab has said Mr Allex was still alive after the rescue attempt.

"It could have succeeded. It should have succeeded. Its consequences weigh heavily and I take full responsibility for the operation, because it is also a message from us: France cannot allow its citizens to be detained," French President Francois Hollande said yesterday.

Admiral Edouard Guillaud, France's military chief of staff, said officials there believed al-Shabab's announcement was propaganda.